Tumor hepatocytes and basement membrane-Producing cells specifically express two different forms of the endostatin precursor, collagen XVIII, in human liver cancers

Endostatin is an endogenous inhibitor of angiogenesis and tumor growth in mice, which may be generated by proteolytic cleavage of collagen XVIII. In normal tissues, 2 variants of the endostatin precursor, namely the SHORT and LONG forms, regulate tissue specificity. We analyzed 53 human liver biopsies (18 hepatocellular carcinomas, 16 metastases of colorectal cancer, 3 cholangiocarcinomas, and 16 controls) by RNA dot blots, double-labeling immunohistochemistry, and in situ hybridization, using common and variant-specific probes. Tumor hepatocytes expressed the LONG form, whereas cholangiocarcinoma cells expressed the SHORT form, which was deposited in tumor basement membranes. Metastatic colorectal carcinoma cells did not express collagen XVIII. In the stromal compartment of primary and metastatic cancers, myofibroblasts and vascular endothelial cells expressed the SHORT form. Both basement membrane components, collagen IV and the SHORT collagen XVIII form, were codistributed and their mRNA levels strongly correlated (R =.75, P <.001). In addition, freshly isolated human hepatocytes expressed the LONG form and culture-activated stellate cells the SHORT form. Moreover, the full-length LONG form is a plasma protein. Thus, the LONG form is a hepatocyte-specific variant, and the SHORT form is a major component of the tumor extracellular matrix in primary and metastatic liver cancers. In the clinical context, the global expression of the endogenous endostatin precursor, collagen XVIII, in liver cancer results from the combined expression profiles of tumor cells, stromal cells, and nontumor hepatocytes at the advancing edge of the tumor, particular to each type of cancer.     

Expression of collagen XVIII mRNA in rat liver fibrosis]

OBJECTIVE: To measure and study quantitatively the collagen XVIII mRNA in normal and fibrotic rat livers. METHODS: We used ribonuclease protection assay to investigate the collagen XVIII mRNA expression in rat liver fibrosis induced by complete bile duct occlusion (BDO). The expression level of procollagen 1 (XVIII) mRNA was compared with that of procollagen 1 (I) and tissue inhibitor of metalloproteinase 1 (TIMP1). RESULTS: mRNA levels of procollagen and TIMP 1 increased 20- and 4-fold in BDO rat livers, respectively. In contrast, hepatic procollagen 1 mRNA level increased only 1.8-fold in fibrotic rat livers. CONCLUSION: C XVIII mRNA is upregulated slightly in liver fibrosis, which is probably correlated with the fact that CXVIII is mainly expressed by hepatocytes.     

Zinc-dependent dimers observed in crystals of human endostatin

The crystal structure of human endostatin reveals a zinc-binding site. Atomic absorption spectroscopy indicates that zinc is a constituent of both human and murine endostatin in solution. The human endostatin zinc site is formed by three histidines at the N terminus, residues 1, 3, and, 11, and an aspartic acid at residue 76. The N-terminal loop ordered around the zinc makes a dimeric contact in human endostatin crystals. The location of the zinc site at the amino terminus, immediately adjacent to the precursor cleavage site, suggests the possibility that the zinc may be involved in activation of the antiangiogenic activity following cleavage from the inactive collagen XVIII precursor or in the cleavage process itself.     

Differential regulation of constitutive androstane receptor expression by hepatocyte nuclear factor4alpha isoforms

Constitutive androstane receptor (CAR; NR1I3) controls the metabolism and elimination of endogenous and exogenous toxic compounds by up-regulating a battery of genes. In this work, we analyzed the expression of human CAR (hCAR) in normal liver during development and in hepatocellular carcinoma (HCC) and investigated the effect of hepatocyte nuclear factor 4alpha isoforms (HNF4alpha1 and HNF4alpha7) on the hCAR gene promoter. By performing functional analysis of hCAR 5'-deletions including mutants, chromatin immunoprecipitation in human hepatocytes, electromobility shift and cotransfection assays, we identified a functional and species-conserved HNF4alpha response element (DR1: ccAGGCCTtTGCCCTga) at nucleotide -144. Both HNF4alpha isoforms bind to this element with similar affinity. However, HNF4alpha1 strongly enhanced hCAR promoter activity whereas HNF4alpha7 was a poor activator and acted as a repressor of HNF4alpha1-mediated transactivation of the hCAR promoter. PGC1alpha stimulated both HNF4alpha1-mediated and HNF4alpha7-mediated hCAR transactivation to the same extent, whereas SRC1 exhibited a marked specificity for HNF4alpha1. Transduction of human hepatocytes by HNF4alpha7-expressing lentivirus confirmed this finding. In addition, we observed a positive correlation between CAR and HNF4alpha1 mRNA levels in human liver samples during development, and an inverse correlation between CAR and HNF4alpha7 mRNA levels in HCC. These observations suggest that HNF4alpha1 positively regulates hCAR expression in normal developing and adult livers, whereas HNF4alpha7 represses hCAR gene expression in HCC.     

Absence of collagen XVIII in mice causes age-related insufficiency in retinal pigment epithelium proteostasis

Collagen XVIII has the structural properties of both collagen and proteoglycan. It has been found at the basement membrane/stromal interface where it is thought to mediate their attachment. Endostatin, a proteolytic fragment from collagen XVIII C-terminal end has been reported to possess anti-angiogenic properties. Age-related vision loss in collagen XVIII mutant mice has been accompanied with a pathological accumulation of deposits under the retinal pigment epithelium (RPE). We have recently demonstrated that impaired proteasomal and autophagy clearance are associated with the pathogenesis of age-related macular degeneration. This study examined the staining levels of proteasomal and autophagy markers in the RPE of different ages of the Col18a1 ^?/? mice. Eyes from 3, 6–7, 10–13 and 18 months old mice were enucleated and embedded in paraffin according to the routine protocol. Sequential 5 μm-thick parasagittal samples were immunostained for proteasome and autophagy markers ubiquitin (ub), SQSTM1/p62 and beclin-1. The levels of immunopositivity in the RPE cells were evaluated by confocal microscopy. Collagen XVIII knock-out mice had undergone age-related RPE degeneration accompanied by an accumulation of drusen-like deposits. Ub protein conjugate staining was prominent in both RPE cytoplasm and extracellular space whereas SQSTM1/p62 and beclin-1 stainings were clearly present in the basal part of RPE cell cytoplasm in the Col18a1 ^?/? mice. SQSTM1/p62 displayed mild extracellular space staining. Disturbed proteostasis regulated by collagen XVIII might be responsible for the RPE degeneration, increased protein aggregation, ultimately leading to choroidal neovascularization.     

Endostatin influences endothelial morphology via the activated ERK1/2-kinase endothelial morphology and signal transduction

Endostatin, the proteolytic fragment of collagen XVIII, is known to be a potent inhibitor of angiogenesis. However, to date, only limited knowledge exists with regard to the effects of endostatin on vessel morphology and the underlying signaling pathway. The aim of the present work was therefore to determine the impact of endostatin and its collagen XV analogue restin on vessel development during wound healing and embryonic angio- and vasculogenesis. Time lapse experiments and electron microscopy demonstrate similar morphological changes evoked by endostatin and the ERK1/2-kinase inhibitor PD98059. Furthermore, we show that ERK1/2 phosphorylation, a crucial signaling event in vascular morphogenesis, is regulated by endostatin via the protein phosphatase 2A PP2A. These findings provide new insight into a key signaling pathway of vascular remodeling evoked by a matrix-derived factor.